Designers of power output devices, e.g., automobile, aircraft and locomotive engines, are faced with a myriad of competing design criteria that result in various design compromises. For example, if it is desirable to optimize power output, a compromise in engine torque is often necessary. Similarly, if one desires optimal energy conversion, e.g., specific fuel consumption, a designer must choose an appropriate air-standard combustion cycle to match this requirement. In yet other examples, if it is desirable to optimize horsepower and torque, one may need to accept weight and/or fuel consumption penalties.
While there are literally thousands of internal combustion engine designs and variations thereof, all employ certain fundamental or basic principles. These include the intake of a combustible mixture, e.g., oxygen and gasoline, into a working area, compression of the combustible mixture, ignition of the mixture to effect its expansion, capturing the energy of the expansion to produce work, and expelling combustion by-products into an exhaust system so as to prepare the working area for a subsequent cycle.
Internal Combustion Engines (ICEs)
Commercially successful internal combustion engines include reciprocating-piston, rotary, and gas-turbine engines. Following is a brief discussion of each followed by the advantages and disadvantages of each. Applications of each are also discussed.
Traditional reciprocating internal combustion engines employ the reciprocating motion of a piston/cylinder to perform the functions described above. Linear motion of the piston is translated into rotational motion by means of a piston rod that is articulately mounted to the underside of the piston at one end thereof and pivotally mounted at the other end to an eccentric portion of a crankshaft. Generally, these internal combustion engines employ the Carnot two-stroke, Otto four-stroke or Diesel two and/or four-stroke air-standard cycles.
The Carnot two-stroke engine typically employs a piston/cylinder arrangement wherein the cylinder comprises inlet and exhaust ports located on opposite sides of the cylinder walls. The inlet port is located slightly above the lowest point of piston travel, e.g., bottom dead center, and the exhaust port is located on the opposing side about a midpoint relative to piston travel. The combustible mixture is first introduced into the cylinder chamber as the piston uncovers the inlet port. The downward motion of the piston from the prior stroke, causes the combustible mixture (located within the housing) to be pressurized thus being forced into the cylinder chamber. As the piston moves upwardly and past the exhaust port, the combustible mixture is compressed and ignited upon reaching the top of the piston's stroke, i.e., top dead center. The expansion of the combustible mixture produces a downward power stoke and, upon passing the exhaust port, begins to be expelled from the cylinder chamber. As the piston travels downward yet further, the combustible mixture is pressured by the underside of the piston (traveling downwardly) and is injected into the cylinder as the piston passes the inlet port. This injection of newly-introduced combustible mixture further augments the expulsion of exhaust gases through the exhaust port. The Carnot two-stroke produces a power stroke for every two strokes of the piston, or once per revolution, i.e., of the drive shaft.
The Otto four-stroke engine typically employs a piston/cylinder arrangement wherein the inlet and exhaust ports are located at the top or uppermost portion of the cylinder and are operated, i.e., opened and closed, by means of cam-driven valves. The combustible mixture is first introduced upon opening the inlet port whereby the downward motion of the piston generates a vacuum for drawing the combustible mixture into the cylinder. The next stroke is the compression stroke, wherein the inlet and exhaust valves are both closed and the combustible mixture is compressed as the piston traverses upwardly. At or near the top of the piston stroke, the air-gas mixture is ignited thus initiating its expansion and downward motion of the piston. Upon completion of the downward or power stroke, the exhaust port is opened such that the subsequent upward stroke of the piston expels the combusted gases outwardly into an exhaust manifold. The Otto four-stroke cycle produces a power stoke for every four stokes of the piston, or once every two revolutions of the drive shaft.
More recent innovations include the Wankel Rotary engine which employ the eccentric motion of a polygonal-shaped rotor within a substantially elliptically-shaped, or more accurately, a epitrochoidally-shaped housing or chamber. The apexes of the polygonal-shaped rotor create discrete chambers which, upon rotation of the rotor within the housing, pass inlet ports, ignition spark plugs and exhaust ports. More specifically, as the rotor passes the inlet port, the combustible mixture is introduced within one of the chambers. As this chamber rotates approximately 90 degrees, the mixture is compressed against a wall of the epitrochoidally-shaped housing. Ignition or spark plugs are located at this angular position and, upon ignition, the combustible mixture expands causing the rotor to be driven approximately 120 degrees within the housing. An exhaust port is located at the next rotational position and the combusted gases are expelled from the chamber. Similar to the compression stage of the Wankel cycle, the chamber is caused to be reduced in volume, i.e., against the wall of the epitrochoidally-shaped housing, thereby expelling the exhaust gases outwardly.
In conventional turbine engines, a compressor section is used to compress air into a combustion chamber. Fuel is introduced into the chamber and ignited to expand the air-gas mixture. Turbine vanes capture the energy of the expanded air-gases to turn a turbine shaft which also drives the compressor section to continue the combustion cycle. Generally, this form of internal combustion engine employs the Brayton air-standard cycle. From its brief description, it will be appreciated that the turbine engine is, perhaps, the most elegant, however, it too has disadvantages which limits its application.
Advantages/Disadvantages & Practical Applications of Reciprocating Piston ICEs
The following is a brief examination of reciprocating piston ICEs in terms of their properties, performance, and practical applications. Inasmuch as the Wankel rotary and turbine engines are not widely employed or have specific/limited applications, these will only be mentioned in terms of there need for gear reduction apparatus to lower output velocities to useable speeds. Furthermore, these engine designs represent a significant departure from the elements and teachings of this invention.
A two stroke air-standard cycle (or Carnot Cycle) delivers a power stoke with each revolution as compared to the four-stroke cycle which delivers a power stroke with every two revolutions. Consequently, reciprocating ICEs employing a two stroke cycle can deliver twice the power of a four stroke. Two stroke engines do not require valves and the associated mechanisms for the intake of fuel and exhaust of combusted gases. Four stroke engines, on the other hand, require a complex array of cam-driven valves for intake and exhaust. ICEs employing a two stroke cycle can operate at any orientation, which can be important in applications wherein the powered-vehicle or device pitches or rolls such as an acrobatic fixed-wing aircraft, helicopter or chainsaw. Engines employing a four-stroke cycle require that oil be delivered from a gravity-based sump. Consequently, four stroke engines typically are designed with the forces of gravity in mind. Two-stroke engines, therefore, offer simplicity and a significant power-to-weight ratio as compared to many four-stroke engine designs.
Disadvantages of the two-stroke air-standard cycle generally involve wear, fuel efficiency and pollution. The lack of a dedicated lubrication system typically results in a high rate of component wear. Further, two-stroke reciprocating engines, which employ a conventional crankshaft, also experience accelerated wear of the piston. To better understand this phenomena, it should be appreciated that the eccentricity of the crankshaft causes the piston rod to be oriented off-axis relative to the piston/cylinder axis. As such, a lateral component of the resultant force vector imposes high frictional forces between the piston and cylinder. Consequently, the piston rings wear, pressure is reduced i.e., causing blow-by, and fuel efficiency decreased. Other disadvantages are simply due to the way fuel is burned (or not burned) in two stroke engines. For example, the exhaust phase of the cycle is, at least in part, combined with the fuel intake and compression phase of the cycle. Consequently, exhaust gases are intermixed with a fresh charge of air-gas, hence, the mixture for ignition is non-optimum, i.e., contaminated by exhaust gases from the previous stroke. Similarly, inasmuch as the intake and exhaust occur nearly simultaneously, but along opposite sides of the cylinder, fresh fuel may be exhausted before ever being compressed and ignited. Consequently, two-stroke engines are not highly fuel efficient.
The principle advantage to a four stroke air-standard cycle (or Otto cycle) relates to fuel efficiency. More specifically, four stroke engines employ a stroke entirely dedicated to the exhaust of combusted gases, hence, four stroke engines burn cleaner and more efficiently that two stroke engines. That is, combusted gases do not intermix with a fresh charge of air-fuel in or during the compression/ignition stroke. Furthermore, four-stroke engines can have independent/dedicated oil and fuel reservoirs, i.e., do not use a gas-oil mixture, hence four stoke engines experience less wear and are less costly to operate.
The disadvantages of four stroke engines have been discussed above, i.e., when being compared to a two stroke engine, however, suffice it to say that four stroke engines deliver significantly less power output than two stroke engines.
Diesel two and four stroke cycles have the same advantages and disadvantages as those discussed above in connection with the Carnot and Otto air-standard cycles. Diesel engines do, however, allow high compression ratios inasmuch as the flash point (i.e., the temperature at which the fuel ignites) of Diesel fuel is substantially higher than conventional gasoline fuel. While this can offer the advantage of high power output, Diesel fuel contains less energy than gasoline (on a BTU/in3 or volumetric basis) and does not produce the same power output when compared to gasoline bumming engines. Generally, the advantage of Diesel engines relates to the low cost of Diesel fuel and the relatively high efficiency with which it burns.
All of the above air-standard cycles and engine designs operate efficiently at relatively high rotational speeds. For example, a gas turbine engine is typically efficient at about ten-thousand (10,000) RPM. Four-stroke automobile engines are efficient within a range of about fifteen hundred to three thousand (1,500 to 3000) RPM. This is also true for the Wankel rotary engine. Typically, such rotational velocities are orders of magnitude above useful speeds to, for example, drive automobile tires, helicopter rotors, ship propellers etc. Consequently, all require speed reduction devices, e.g., transmissions, to lower and control the speed of output drive shafts. It will be appreciated that such devices add significant weight, require periodic maintenance, and are costly to fabricate, operate and maintain.
Other disadvantages of ICEs of the prior art relate to the weight distribution of conventional designs and to a lack of balanced torque output. With respect to the former, the center of gravity (C.G.) of prior art ICEs is frequently offset with respect to the output shaft axis. While this does not present difficulties in many applications, in other applications, such as a compound helicopter, it is beneficial to have the engine C.G coincident with the output drive shaft axis. For example, helicopters typically are designed such that the turbine engines are juxtaposed relative to the helicopter transmission. Despite the output orientation of the turbine engine (which faces forward), a rather elaborate bevel gearing system is employed to ensure that the center of gravity of the turbine engine lies in the same plane (normal to the longitudinal axis of the helicopter). As such, this drive-train configuration is non-optimal in terms of weight and is highly mechanically complex.
With respect to the latter, helicopters typically employ anti-torque devices to counter-act the torque developed in the fuselage as a result of the high torque required to drive the main rotor system. Conventional anti-torque devices employ tail rotors or propulsive thrusters to generate a force vector equal and opposite to the engine-generated moment vector, i.e., at a calculated distance from the rotational axis. As such these devices, which include tail cone structure, tail drive shafts, tail rotor gearboxes also add unnecessary weight.
A need, therefore, exists for an ICE which maximizes energy conversion, eliminates the need for intermediate speed reduction devices, e.g., speed reducing transmissions, improves performance, enhances reliability, reduces weight and minimizes mechanical complexity.